It started with a Facebook "friend" request.

She was 18 at the time, and didn't imagine that clicking "accept" would start her on a path to four years of prostitution across the country. "Nina" is a pseudonym; CNNMoney agreed to change the names of the victims in this article to protect their privacy.

Upper middle-class and college-bound, Nina had her plans derailed in her senior year of high school after her mother was sentenced to two years in prison for financial crimes. Lonely and looking online for male attention, she started messaging back and forth with a man who said he was falling for her. They talked about trips they'd take together as a couple, and about marriage, maybe kids.

"He sold me the biggest dream in the world," she says. "I thought he really did like me and we were going to live this fairy-tale life together."

They exchanged online messages for about a month. That September, while Nina's friends went off to college, she traveled the two and half hours from home to meet her Facebook beau in person.

The fairy tale ended fast. Almost immediately after she arrived in Seattle, he dropped her off on a street where prostitutes troll for customers and told her she was going to "catch dates."

Many would have run, but Nina says her deteriorating family life left her with a sense of desperation. She was smitten, and willing to do anything for the man she thought loved her. So she stayed.

Keeping the attention of her "boyfriend" required selling herself for sex, Nina learned. He was a pimp -- and she was one of a growing number of women recruited on social networks for sex trafficking.

There are no hard statistics on the scope of the problem. Law enforcement officials don't track how sex workers are recruited into the field, and unless the victims are underage, prostitution is typically a low-priority crime.

But recent prosecutions in California, Virginia and Washington, along with interviews CNNMoney conducted with victims and those investigating these crimes, illustrate how social networks are helping traffickers lure in victims like Nina.

"Pimps are professional exploiters," says Andrea Powell, executive director of Fair Girls, an organization that helps victims of sex trafficking. "Often they're just spamming a whole bunch of girls with messages like, 'Hey, you look cute. I could be your boyfriend.'"

That's one way Justin Strom -- aka "J-Dirt" -- recruited the high-school girls he and his followers trafficked in Alexandria, Va., an affluent suburb on the outskirts of Washington, D.C. For six years, the members of Strom's "Underground Gangster Crips" gang operated a prostitution ring that ensnared at least eight 16- and 17-year olds, according to court documents.

The girls were rented out to five to 10 customers each on a typical night. The going rate was around $30 for 15 minutes of sex.

Social networks were among Strom's preferred hunting grounds.

The group "searched Facebook for attractive young girls, and sent them messages telling them that they were pretty and asking if they would like to make some money," one witness told a Federal Bureau of Investigation agent investigating the case. The court records include a trail of those messages.

Facebook messages from Rain Smith

Messages provided by U.S. Department of Justice. Visualization created by CNNMoney.

If a girl expressed interest, a gang member would arrange to meet up. At that point, participation stopped being voluntary.

One 17-year-old solicited on Facebook allowed Strom to pick her up in his car at her home, but when he spelled out what he expected, she told Strom she wanted out. In response, he "slammed her head against the window of the vehicle," forced her to ingest cocaine, and slashed her arm with a knife, according to court documents.

That night, he took her to an apartment complex and rented her out to 14 men. The encounter netted Strom $1,000. It left the victim with a collection of physical scars.

"He's a con artist, a monster and a manipulator," another victim testified at his sentencing. "I was brainwashed into believing that having sex with men for money was normal, an everyday thing."

An FBI operation shut Strom's gang down last year, and in September he was sentenced to 40 years in prison. Four of his associates were also convicted.

But FBI agent Jack Bennett, the bureau's cybercrimes chief in San Francisco, says Strom's tactics are becoming more common. Part of the problem, he says, is that minors will accept friend requests from strangers just to appear to be popular. Photos, personal information, and friend lists are then out in the open.

Pimps "start looking for the cracks where they can fill the holes, whether it be a father figure or a boyfriend," Bennett says.

Some are even more direct.

"Lisa," a 21-year-old who was trafficked for most of her teenage years before escaping in mid-2012, gets daily messages on social networking sites from traffickers trying to reel her back in. Many don't even hide their intentions.

"If it's a 'P' beside their name, that stands for pimp," Lisa says. On any given day, she gets a steady stream of messages from unfamiliar men whose last names are just "P."

'Old tricks with new tools'

Powell, the advocate who runs Fair Girls, says she's seen girls recruited from almost every social network that exists. Facebook and Tagged are two of the most common, she says, but even more limited sites like Twitter and Instagram get used for solicitation. The FBI's case against Strom cites DateHookup and MySpace, in addition to Facebook, as sites his gang targeted.

In a recent Seattle case involving multiple juveniles, Facebook was used to recruit one of the victims. The two defendants were charged in Pierce County, Wash., in November.

"What you're really seeing here with Facebook, and other social networking sites, is old tricks with new tools," says Pierce County prosecutor Mark Lindquist.

The Polaris Project, which runs a sex-trafficking help hotline, works with tech companies to educate them on how their technology is being used to facilitate trafficking, and how they can help stop it.

"They're most interested in understanding exactly how the criminal networks are operating, and they want to know the modus operandi of the traffickers," says Bradley Myles, executive director of Polaris.

Facebook(FB) reacts swiftly to reports of illicit activity and quickly takes down questionable content when it's flagged, according to Myles and other advocates.

The company says it takes human trafficking very seriously.

"While this behavior is not common on Facebook, we have implemented robust protections to identify and counter this activity," a company representative told CNNMoney in a written statement. "We have zero tolerance for this material and are extremely aggressive in preventing and removing exploitative content. We've built complex technical systems that either block the creation of this content, or flag it for review by our team of investigations professionals."

But algorithms can't catch everything, and pimps are skillful social engineers.

During down time, Nina's pimp browsed through her Facebook friends, sending friendship requests using her profile and messaging women he thought "looked the part."

Strom used similar tactics, relying on women he controlled to reach out to new prospects. He also sent hundreds of messages himself to teenagers, with pitches like: "I work with girls that dance nude do partys dates one on ones and more does any of that interest you."

Calvin Winbush, who calls himself "Good Game," ran a prostitution business out of Ohio. He was sentenced in August to 14 years in prison for trafficking minors across state lines for prostitution. Winbush described himself as an "international player" on his Facebook page, and recruited heavily with messages like: "Call me soon as u get this love so we can chop it up and get better acquainted..."

Facebook messages sent by Calvin Winbush

What up Bri? Call me soon as u get this love so we can chop it up and get better acquainted.. Yo future everythang a real true pappy dats gone keep u happy!!

Call me…Good Game I’m n Memphis. I wanna pick u up and take u wit me so we can get sum paper!

Call me if u all grown up and ready 4 a good pappy dats gone keep u happy!!

Messages provided by U.S. Department of Justice. Visualization created by CNNMoney.

That kind of approach works more often than parents would like to believe.

"There's no high school that's immune to this," Ken Cuccinelli, Virginia attorney general, said in a press conference unveiling the charges against Strom. "It demands increased vigilance by both parents and law enforcement into the activities that are occurring across those social media lines."

The FBI, which is often on the front lines of investigating these cases, has a tip sheet on its website to help parents protect their children on social networks.

The agency recommends that parents monitor their kids' online profiles and postings -- a controversial step in many households, but one the agency thinks is essential. It also recommends that parents educate their kids about how broadly the messages and photos they post online can spread. Teenagers don't always realize that they can't "take back" texts and images.

"I've talked to parents who say, 'Hey listen, my son has to set up my computer 'cause I just don't know,'" says the FBI's Bennett. "That's not an excuse anymore. You've got to know, because it's your child's life and their well-being depends on this."

Breaking free

Nina describes being raped, beaten with a pistol, and, once, locked in a closet for 24 hours. Beyond the physical threats, shame kept her from running away.

"I didn't want to tell my parents, 'Ya know, this is what I'm doing,'" she says. "How am I going to explain that to my father? That wasn't an option for me at all."

Nina bounced through a series of different pimps, eventually ending up "working" for a trafficker who took away her ID and forced her to dance -- and more -- at strip clubs and in hotel rooms.

A massive raid by local police and the FBI shut down his operation about a year ago. Without that, Nina says she could still be working for him today. Advocates at Fair Girls are helping her rebuild her life. She's planning to begin college in the fall.

Both Nina and Lisa still maintain accounts on the social networks on which they were recruited, mainly to keep in touch with friends and family. Both receive daily messages from pimps.

They no longer respond.

"I want to get my life together," says Lisa, who is working to earn her GED. "If I start school, I probably won't have a Facebook page."